4 8 CHEMICAL CONSTITUENTS OF BOD Y AND FOOD. 
considerable change in the proteicl molecule ; indol and skatol can 
hardly he considered to be simple cleavage products of the proteid 
molecule (see p. 29). 
Idebermann's test 1 is performed by precipitating the proteid by 
alcohol, and then heating the washed precipitate with strong hydrochloric 
acid. The result is a violet-blue colour. The reaction is not given by pure 
peptone. 2 It is also not given by any of the aromatic putrefactive products 
of proteid, nor by a large number of other cleavage products of proteid 
which Pickering worked with. Its cause is therefore at present unknown. 
Krassers reaction. 3 — Alloxan in solution stains proteid matter a 
brilliant red. It reacts in the same way with asparagin, aspartic acid, 
and tyrosine. The reaction is probably connected with the presence 
of amido groups. 
Piotrowski's reaction. 4 — If a few drops of dilute copper sulphate 
solution are added, and then excess of strong solution of caustic soda 
and potash, a violet solution is the result. If ammonia is used instead, 
a blue solution is formed. 
In the case of the proteoses and peptones, the result is a rose-red solu- 
tion with potash 5 and a reddish violet with ammonia. As the same colour 
is given by the decomposition product of urea called biuret, 6 the test 
is often called the biuret reaction. (2CON,H 4 - NH 3 =C 2 2 N 3 H 5 ). Biuret 
yields, on decomposition, compounds which contain cyanogen; for instance, 
by heat it is split into ammonia and cyanuric acid, (CN^B^C^. Biuret, 
cyanuric acid, xanthine, hypoxanthine, sarcosine, hydrocyanic acid, all give 
similar reactions to the proteids. Gnezda 7 considered it probable that the 
biuret reaction was due to a cyanogen radicle, and that the cyanogen in 
albumin and peptone is differently combined, corresponding to the similar 
differences in cyanuric and hydrocyanic acid respectively. Pickering, 8 
however, concludes, that the radicle in cpiiestion is not CN but CONH. 
The related metals, nickel (Gnezda) and cobalt (Pickering) give correspond- 
ing colour reactions, which may be summarised in the following table : — 
Proteid. 
Copper Sul- 
phate and 
Ammonia. 
Copper Sul- 
phate and 
Potash. 
Nickel Sul- 
phate ami 
Ammonia. 
Nickel Sul- 
phate and 
Potash. 
Cobalt Sul- 
phate and 
Ammonia. 
Cobalt Sul- 
phate and 
Potash. 
Native proteids") 
(alb n min s, j 
globulins, and | 
nunleo - pro- j 
teids) J 
Products of pro-"\ 
teolysis (pro- 1 
teosesandpep- 1 
tones) J 
Blue 
Violet 
Violet 
Rose -red 
Nil 
Yellow 
Yellow 
Orange 
Nil 
Nil 
Heliotrope- 
purple 
Red -brown 
Pickering found that when a cobalt salt has entered into the proteid 
1 Jdhresb. ii. d. Fortschr. d. Thier-Chcm., Wiesbaden, Bd. xvii. S. 8 ; Chan. Ccntr.-BL, 
Leipzig, 1887, Nos. 18 and 25. 
2 Le Nobel, Jahrcsb. it. d. Fortscltr. d. Thier-Chem. t Wiesbaden, Bd. xvii. S. 3. 
3 Monalsh. d. Ckem., Wien, Bd. vii. S. 673. 
4 Sitzungsb. d. Tc. AJcad. d. Wissensch., Wien, Bd. xxiv. S. 335. 
3 Briicke, Monatsh. d. Chain., Wien, Bd. iv. 
6 Wiedemann, Ann. d. Pkys. u. Chem., Leipzig, Bd. lxxiv. S. 67. 
7 I 'nn\ Roy. Soc. London, 1889, vol. xlvii. p. 202. 
s Juurn. Physiol., Cambridge and London, 1893, vol. xiv. p. 347. 
